It's kind of like comparing feet and inches to meters and centimeters... it's two different scales.
I think the PS-ratings are based on the ABRSM grade-levels, which go from 1-8. Beyond grade 8 they have three diploma-levels. Those pieces are just given the grade 8+ on PS.Henle created their own rating-system that goes from 1-9, where 9 is given to the absolute hardest pieces, corresponding to ABRSM's final diploma-level. As far as I can tell, it looks like grade 5 on the Henle-system is roughly equivalent to grade 8 on ABRSM.It's kind of like comparing feet and inches to meters and centimeters... it's two different scales.